The pressurized water heated in contact with the fuel elements of a nuclear reactor passes in fact through a generator of this kind, circulating in a bundle of U-tubes and transmitting its heat to a secondary fluid, which is thus vaporized inside the generator. The bundle of U-tubes is supported by a tube plate which, with the bottom of the steam generator, delimits a water container separated into two compartments for the inflow and outflow, respectively, of the pressurized water each of these compartments incorporating an orifice which connects it to the primary circuit of the reactor and whose diameter must be large to allow high flow rates.
The considerable risk of corrosion of the tubes in the bundle and the danger entailed by communication between the primary and secondary circuits via holes formed in these tubes through the action of this corrosion make it necessary to subject this bundle of tubes to frequent maintenance and checking operations. It must therefore be possible both to gain access to each of the compartments of the water container, and to isolate these compartments from the primary circuit of the reactor.
Access to the compartments is gained via manholes which must have the smallest possible dimensions, and consequently usually a diameter just sufficient for the normal passage of the operator. During normal operation of the reactor these access points must in fact be leaktightly closed, while the primary fluid contained in the water container is under very high pressure.
The isolation of the water container implies leaktight closure of the orifice making the connection to the primary circuit of the reactor, even though the circulation of this fluid has stopped. However, this orifice has a diameter substantially larger than that of the manhole giving access to the water container and constituting the only passage allowing the introduction of closure means, while in addition it must be closed quickly in order to ensure the safety of the operator.
Various types of shutter have therefore been proposed, consisting of a plurality of members articulated to one another and adapted to be folded, but these devices present difficulty in reconciling the requirements of leaktightness at high pressures, speed of installation, low weight, and ease of introduction through an orifice of small dimensions.